1. Technical Field
Exemplary aspects of the present invention relate to a fixing device, an image forming apparatus, and a fixing method, and more particularly, to a fixing device for fixing an image on a recording medium, an image forming apparatus incorporating the fixing device, and a fixing method for fixing a toner image on a recording medium.
2. Description of the Background
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a development device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device may employ an endless belt or an endless film to heat the recording medium. For example, the fixing device may include an endless belt and a pressure roller pressed against the belt to form a fixing nip therebetween. As a recording medium bearing a toner image is conveyed through the fixing nip, the belt and the pressure roller apply heat and pressure to the recording medium, melting and fixing the toner image on the recording medium. The belt is requested to be heated quickly to shorten a first print time taken to output the recording medium bearing the fixed toner image upon receipt of a print job. Additionally, as the image forming apparatus conveys an increased amount of recording media at high speed, the belt is requested to overcome shortage of heat.
In order to shorten the first print time, the fixing device may include an endless film and a pressure roller pressed against a ceramic heater disposed inside the film to form a fixing nip between the film and the pressure roller. As a recording medium bearing a toner image is conveyed through the fixing nip, the film heated by the ceramic heater and the pressure roller fix the toner image on the recording medium under heat and pressure. Since the film is heated by the heater situated at the fixing nip, the film is heated insufficiently at an entry to the fixing nip, resulting in faulty fixing. Accordingly, the film is requested to overcome shortage of heat at the entry to the fixing nip.
To address those requests, the fixing device may employ a tubular, metal thermal conductor disposed inside the belt. The pressure roller is pressed against the metal thermal conductor via the belt to form a fixing nip between the belt and the pressure roller. A heater situated inside the metal thermal conductor heats the metal thermal conductor which in turn heats the belt. As a recording medium bearing a toner image is conveyed through the fixing nip, the belt heated by the metal thermal conductor and the pressure roller apply heat and pressure to the recording medium, fixing the toner image on the recording medium. Since the tubular, metal thermal conductor is disposed opposite the belt throughout the entire circumferential span thereof, the metal thermal conductor heats the belt quickly, thus shortening the first print time and overcoming shortage of heat.
In order to shorten the first print time and save energy further, the belt heated by the heater directly, not through the metal thermal conductor, is proposed. For example, the fixing device may include the belt and a light emitter situated inside the belt. The light emitter emits light onto the belt in a first span in a direction perpendicular to a conveyance direction of the recording medium conveyed over the belt and a second span in the conveyance direction of the recording medium. A light shield produced with a recess through which light from the light emitter travels to the belt is interposed between the light emitter and the belt. A motor moves the light shield in the conveyance direction of the recording medium. A width of the recess in the direction perpendicular to the conveyance direction of the recording medium is within the first span of the belt and variable depending on the position of the recording medium in the conveyance direction thereof.
The light emitter irradiates the belt through the recess having a first heating span corresponding to a first width of a recording medium and a second heating span corresponding to a second width of a recording medium. Since the light shield shields light from the light emitter at a portion other than the recess, the belt is heated in a span corresponding to the first heating span or the second heating span of the recess. As the motor moves the light shield in the conveyance direction of the recording medium, the span of the recess of the light shield changes within the first span of the belt as the recording medium moves in the conveyance direction thereof.
During a fixing job, the light shield shields the belt from the light emitter in a non-conveyance span of the belt where the recording medium is not conveyed, preventing overheating of the non-conveyance span of the belt. However, since the light shield shields the non-conveyance span of the belt from the light emitter, while the fixing device is warmed up from an ambient temperature to a predetermined temperature as the fixing device is powered on, the light shield may render the light emitter to heat the belt unevenly. Accordingly, during a next fixing job, uneven temperature of the belt may cause variation in gloss of the toner image fixed on the recording medium depending on the size of the recording medium used in the next fixing job. Additionally, the light shield shielding the non-conveyance span of the belt may save energy insufficiently and lengthen the first print time.